1. Field of the Invention
This invention relates in general to a method and apparatus for forming well holes. More particularly the invention relates to a method and apparatus for drilling directional boreholes. Still more particularly, the invention relates to forming multiple deviated branch holes from a single location either from an offshore platform or from a single land based rig, into multiple horizontal locations of a pay zone.
2. Description of the Prior Art
Standard practice for producing offshore oil and gas fields calls for drilling multiple wells from a single platform. The multiple wells are deviated out into the pay zone in order to economically develop the field. The standard practice on land is to drill multiple wells into the reservoir and only in unusual situations does it become economically desirable to drill multiple deviated holes out into the formation from a single site. In either case, maximizing the effectiveness of each platform or rig is desirable. In those instances where it is desirable to drill multiple holes from a single site, the production of the field is accomplished by means of directional drilling whereby the drill bit is deflected up to the maximum rate attainable in order to achieve the broadest possible dispersion into the pay zone.
Such deviation allows the wells to fan out from a central location covering a large area of the field, and maximum production is achieved from a single site. However with a shallow pay zone, the directional wells enter the pay zone before achieving sufficient horizontal distance from the platform or rig. The inability to deviate the well sufficiently horizontally because of the shallowness of the pay zone reduces the area coverage from a single site causing many shallow oil and gas fields to be uneconomical to produce, especially in offshore waters where drilling costs substantially exceed land operations.
Another consideration is tight formations or formations which hold high viscosity crudes and require much closer well spacings in order to drain the field. In such cases, if deviated branch holes can be drilled from a single vertical hole, the productivity of each drill site can be increased by permitting better penetration of the target zone. This reduces the number of vertical holes required while at the same time increasing the productivity of the formation into a single well bore.
The prior art has many references to drilling of multiple branch wells from a common vertical well. For example, U.S. Pat. No. 4,396,075 to Wood, et al, illustrates the drilling of a plurality of wells from a vertical shaft by deviating each of the branch wells from the vertical shaft primarily by means of a whip stock drilling guide. U.S. Pat. No. 4,415,205 to Rehm, et al, is another example of producing deviated wells from a central shaft and illustrates the use of a casing which has an internal indexing dog in specific areas which provides "windows" from which some of the branch wells are to be drilled. U.S. Pat. No. 4,402,551, issued in the name of Wood, illustrates the use of a vertical shaft from which a horizontal drain hole is drilled below the cased vertical hole. Likewise, U.S. Pat. No. 4,432,423, issued in the name of Lyons, shows a method and apparatus for drilling a horizontal hole into a producing formation from a vertical shaft.
U.S. Pat. No. 4,431,069 to Dickinson, et al, illustrates a method and apparatus for performing and using a borehole in which a vertical shaft is first produced. A horizontal shaft extends from the vertical shaft and from the horizontal shaft the borehole turns upwardly. Drilling fluid comprising hot acid or basic aqueous or petroleum base solution is used in the drilling process with the aid of an eversible tube. U.S. Pat. No. 2,404,341, issued in the name of Zublin, illustrates a method of producing oil and gas through a deviated bore in which multiple bore holes extend from an initial vertical shaft. The multiple bore holes are illustrated as extending generally outwardly, first downwardly, then upwardly, into a pay zone or hydrocarbon bearing formation. Each of the drilling methods and apparatus disclosed in the references mentioned above have difficulty in reaching sufficiently horizontally from the vertical shaft, especially for shallow formations. For example, one obvious difficulty of drilling a horizontal well from a vertical shaft into a pay zone is to obtain sufficient axial force on a drilling bit in order to drill the horizontal length of hole. Such difficulty is obviously encountered with the use of conventional drilling equipment where the drilling bit is rotated by coupling it to a rotating drill string and is dependent on the limited deviation capability of the string.
The prior art also contains many methods for drilling downard and outward from a vertical bore to achieve increased production such as, by way of example, drain hole drilling. The "drainholes" (holes that turn at right angles to the initial hole and have curvature radii less than 1000 feet) or conventional downwardly directed deviated holes (those holes which have curvature radii less than 100 feet). This has only been modestly successful since pumping systems must then be made to go down each such downward sloping branch to extract the fluid. This added complexity is not always economical nor is it an efficient method for producing the well.
U.S. Pat. No. 4,066,137 issued to Frankle, illustrates apparatus for forming a generally lateral drainhole by which oil from a target formation may be drained to the central shaft. The flame jet apparatus must be removed and reinserted when fuel sources are depleted and in that unconventional flame or "rocket" jetting is used to form the drainhole.